The implementation of a fast track section is a commonly used strategy to improve patient flow in emergency departments (EDs). A fast track section reserves resources (beds, doctors and nurses) for lower acuity patients, and is thus aimed to reduce the wait time and length of stay of these patients. We use a discrete event simulation to investigate the impact of adding a fast track section to an emergency department. We quantify the effect of introducing a fast track on length of stay and waiting time to bed for low and high acuity patients in a crowded ED and compare it to an ED without fast track (Combined ED). We simulate a crowded ED by increasing the patient arrival rate, changing the acuity mix and increasing the time taken for admitted patients in the ED to obtain an inpatient bed (boarding time). We demonstrate that, when compared to a Combined ED with the same number of resources, the introduction of a fast track reduces the wait time to bed for lower acuity patients. However, this comes at the cost of increased waiting time for some higher acuity patients, which is unacceptable in practice. In investigating the solutions to this problem, we find that changing patient prioritization is the most effective way of reducing wait times under crowding. This change in priority does not require the addition of beds, doctors and nurses, and is therefore a cost-effective approach. Finally, we discuss the implications of our results for emergency departments.